Tips for sculpting tile-like terrain
As an expansion of gandalf3's answer, I'll give a quick workflow on how to make tiling easier...

Start with a mirror modifier on your object along both the X and Y axes.


Model the edges of the mesh. Since you have the mirror modifier, it will tile evenly along both the X and Y objects.


Apply the mirror modifier.

Hide the edges of the mesh (Select the edges you want to hide, then press H. To hide in sculpt mode press H (this will bring up box select) and select the very edges of the mesh), and model (or sculpt) the middle part of the rock.
Since you hid the outer edges, they will not be effected when modeling the rest of the rock.


Unhide the hidden edges (Alt+H)


Tile using the Array modifier.